The objective of this study is to determine whether there is an alteration in the pharmacokinetics of morphine or its 3- or 6- glucuronide metabolites when administered as a single intravenous morphine dose followed by an intravenous dose of ondansetron. Secondary objectives are to determine: 1) whether ondansetron alters the pharmacodynamics of morphine, and 2) the elimination half-life of morphine and its 3- and 6- glucuronide metabolites when morphine is administered alone and followed by a dose of ondansetron.